Cartridge case

ABSTRACT

A cartridge casing is formed from a high strength aluminum alloy of the Al-Zn-Mg-Cu type having additionally added thereto up to 0.50 percent by weight silver. On the interior of the casing is a coating, at least 30 Mu m thick, of an elastic synthetic plastic material having a polyethylene or polyurethane precursor.

14 1 Aug. 14, 1973 CARTRIDGE CASE [75] Inventor: Roll Friederlch Weyhmuller,

Greifensee, Switzerland [73] Assignee: Werkzeugmaschlnenfabrlk Oerlikon-Buhrle AG, Zurich, Switzerland [22] Filed: June 9, 1970 [21] Appl. No.: 44,896

52 us. (:1. 102/43 R, 102/43 P 511 1111. c1. ..F42b 5/26 [58] Field of Search 102/38, 42, 43, 43 P, 102/43 F [56] References Cited UNITED STATES PATENTS 2,919,647- 1/1960 Dear et a1. 102/43 R x 3,292,539 12/1960 19611181111 102/43 P 3,144,827 8/1964 Boutwell 102/43 P 3,048,105 8/1962 Schlatter 102/43 R 2,193,573 3/1940 Tackler 102/43 R 3,426,684 2/1969 Jacobsonet a1. 1 102/38 3,498,221 3/1970 Hilton 61 a1. 102/43 R OTHER PUBLICATIONS Ordnance, The Steel Cartridge Case; by K. T. Norris; p. 930-934; May-June 1954.

Primary Examiner-Robert Stahl Attorney-Werner W. Kleeman 2 Claims, 1 Drawing Figure CARTRIDGE CASE BACKGROUND OF THE INVENTION The invention relates to a cartridge or shell case.

Cartridge cases are usually made of materials that are liable to corrode and it is therefore the practice to protect them by electroplating or lacquering.

Care is taken to ensure that the interior of the case is also protected against corrosion sufficiently to prevent the propellant powder in the cartridge case from being contaminated by the products of corrosion of the metal case.

On the other hand it is frequently found that spent cartridge cases contain cracks, due primarily to flaws in the material or to other processing faults. Although they are usually only haircracks which only slightly affect the explosive efficiency of the cartridge, the presence of such cracks is nevertheless undesirable because they permit powder gases to escape into the cartridge chamber and to foul the firearm very considerably. If this happens frequently the firearm itself may suffer corrosion due to the erosive action of the escaping gases.

DESCRIPTION OF THE PRIOR ART Efforts have been made to eliminate cracked cases by introducing careful methods of testing. However, these are expensive and are not absolutely reliable.

Methods have therefore been sought of coating the inside of the case with a layer of material that will prevent the hot and highly corrosive powder gases from blowing out through any cracks in the case. However the electro deposited layers or lacquer films usually applied are severely damaged by the effects of combustion of the powder and they do not give sufi'icient pro' tection. In particular films of lacquer based on epoxy resin are carbonised under the thermal shock of combustion. v

Lacquers have also been tried which exhibit a fairly high temperature stability, but these likewise proved to be unsatisfactory because parts of the lacquer film are burnt off because of the very high stressing of the cartridge case wall that occurs duringfiring of the shell or cartridge. Other inorganic layers, such as phosphate layers, have also proved unsatisfactory.

OBJECTS AND BRIEF DESCRIPTION OF THE INVENTION With the above discussion in mind, it is an object of the present invention to provide a cartridge casing formed of a material which substantially resists the formation of cracks during combustion.

It is a further object of the present invention to provide such a cartridge casing which is coated on the inner surface thereof with an elastic material which will create a seal in any crack which is formed during combustion, thereby preventing escape of powder gases into the cartridge chamber during combustion.

The above objects are achieved in accordance with the present invention by providing the inner surface of the cartridge or shell casewith a' coating of an elastic synthetic plastic material; based on a rubber precursor material to a thickness of at least 30 pm.

It was discovered that a synthetic plastic material of the above kind or lacquers based on these plastic materials, though possessing only the usual thermal stabilities, are in fact extremely resistant to a thermal shock of short duration. Moreover, if these substances possess this property in combination with a particularly high elasticity, as applies to synthetic plastics based on polyethylene or polyurethane, then they are particularly suitable for the contemplated purpose.

The effect of such an inside film may be best described in an illustrative example. When the internal pressure in a cartridge case builds up during the combustion of the powder, the high tangential tension in the case may cause the material to crack whereever there is a weakness. The inside film of highly elastic material that has been applied in a thickness between 30 and um does not burn during the short period of the thermal shock, but the high internal pressure forces it into any existing crack where the elastic material then forms a seal. Although the build-up to the pressure peak is extremely rapid, about 1 ms, the crack forms at the instant of maximum stress or during the rise in stress and therefore precedes the instant of maximum pressure. Consequently the highly elastic plastic film which is forced into the crack experiences the thermal shock for only an extremely short period. Inside the crack only a very small area of the plastic is exposed to the thermal shock so that the volume of highly elastic plastic material that is forced into the crack is sufficient to prevent the powder gases from escaping and from causing the above mentioned undesirable effects in the firearm.

This effect can be further improved by incorporating in the synthetic plastic a fibre structure, for instance in the form of colloidal alumina, which raises the resistance of the plastic to thermal shock and ensures that the described sealing effect is produced even in the case of wider cracks.

An internal protective coating according to the invention is particularly useful if the cartridge cases are made of a light metal alloy, since in such a case the high reactivity of such an alloy with residual oxygen would otherwise create the danger of local cracks initiating an exothermic reaction that would cause the inside of the BRIEF DESCRIPTION OF THE DRAWING The invention will be better understood and objects other than those set forth above, will become apparent when consideration is given to the following detailed description thereof. Such description makes reference to the annexed drawing wherein the single FIGURE thereof depicts a fragmentary sectional view of a portion .of a cartridge case designed according to the teachings of the present invention.

DETAILED DESCRIPTION OF THE INVENTION Now the aforementioned difficulties can be overcome in the following way: In manufacturing a cartridge case from solid bar material a cup is formed in a first operation by forging involving a very high degree of plastic deformation. if in the forming of this cup a billet of high strength light metal alloy is inserted into the die together with a plate of pure aluminium, the latter facing the punch of the press, then the process of forging will result in the cup being formed from both these materials. On the inside of the cup the two materials will be cold bonded, provided the surfaces of the two materials had previously been metallically bared where they are to combine, and provided the degree of plastic deformation is sufficient. Thus a relatively thin layer of pure aluminum, about l to 100 pm thick, is formed on the inside of the cartridge case. This layer will be metallically bonded to the rest of the shell of the hollow case. During a following surface treatment by anodic or chemical oxidation to provide a base for the adhesion thereto of the lacquer, the pure aluminum surface provides a highly satisfactory base.

The liability of cartridge cases made of high strength light metal alloys to suffer from stress corrosion is accentuated by the crisping of the projectile into the mouth of the cartridge case. The crisping operation involves cold working the material in the neighborhood of the mouth of the case which is particularly exposed and where stress corrosion is most likely to occur. Since the projectile usually contains highly sensitive explosives, no thermal treatments for stress relief can be performed after the end of the case has been crisped around the projectile. It is therefore important ab initio to choose a material that is the least likely to suffer from stress corrosion. Such a material is an alloy of the Al-Zn-Mg-Cu type wherein some of the copper is replaced by an addition of silver and the other components of the alloy are adjusted to this change.

Furthermore, for application in cartridge cases the alloy may be modified by limiting its magnesium content to a maximum of 2.5 percent and its copper content to 1.4 percent in association with 0.1 0.3 percent chromium and by keeping the percentages by weight of iron and silicon below 0.5 percent each. An addition of silver up to 0.5 percent by weight is provided.

The alloy should contain as little impurity metal as possible. The chromium in conjunction with the copper and silver contents are necessary to achieve a particularly high resistance to stress corrosion. Continuously cast billets may be used which are solution heat treated before being formed in the press. The alloy may also contain up to 0.2 percent manganese as well as 0.005 to 0.015 percent beryllium for the suppression of sparks.

The liability to exhibit stress corrosion, particularly in the area of the mouth of the cartridge case, can be reduced if the strength of the wall of the case at its mouth is slightly lowered by heat treatment. This is usually also permissible in cartridge cases for high performance rapid firing guns because the strength is principally needed in the region of the thicker cartridge base where the case is exposed to the wearing forces of the firearm and to the forces applied by the ejector devices.

Referring now to the single FIGURE, there is illustrated therein a cartridge casing 10 made of the aforementioned high strength aluminium alloy of the Al-Zn- Mg-Cu type which contains silver. This cartridge casing 10 is provided at its inner surface with means for porviding a seal of any cracks, such as indicated by the crack 12, formed during combustion in the casing. Such sealing means will be seen to be in the form of a coating 14 on the inner surface of the casing 10, this coating consisting of an elastic synthetic plastic material based on polyethylene or polyurethane. This coating 14 is ina thickness of at least 30 pm to provide a good seal of any-cracks formed during combustion, thereby preventing that powder gases resulting from combustion can escape through such cracks. Further, the plastic material can have incorporated therein a fiber structure, generally indicated at 16, in the form of colloidal alumina.

While there is shown and described present preferred embodiments of the invention, it is to be distinctly understood that the invention is not limited thereto but may be otherwise variously embodied and practiced within the scope of the following claims. Accordingly, I claim:

1. An ammunition cartridge casing formed of a high strength aluminum alloy of the Al-Zn-Mg-Cu type and containing silver, said silver being in an amount not exceeding 0.5 percent by weight, the improvement comprising the combination with said alloy containing silver and resistant to the formation of cracks in said casing during combustion, of a coating on the inside surface of the cartridge casing consisting essentially of an elastic synthetic plastic material selected from the group consisting of polyethylene and polyurethane in a thickness of at least 30 m for providing a seal of any cracks formed during said combustion, to prevent the escape through said cracks of powder gases resulting from said combustion.

2. An ammunition cartridge casing according to claim 1, wherein said plastic material has incorporated therein a fiber material consisting of colloidal alumina. i i i 4 

2. An ammunition cartridge casing according to claim 1, wherein said plastic material has incorporated therein a fiber material consisting of colloidal alumina. 